Compressibility of gas hydrates

Area of Science:

• Materials Science
• Geophysics
• Physical Chemistry

Background:

• Gas hydrates are crystalline solids containing water molecules forming cages around guest molecules.
• Understanding their mechanical properties under pressure is crucial for geoscience and materials applications.

Purpose of the Study:

• To experimentally determine the pressure dependence of unit cell parameters for ethane, xenon, and tetrahydrofuran+xenon gas hydrates.
• To calculate the bulk modulus of these hydrates using the Birch-Murnaghan equation.
• To analyze the factors influencing the bulk modulus of gas hydrates.

Main Methods:

• Experimental measurement of unit cell parameters for gas hydrates under varying pressures (0-3 GPa).
• Data fitting using the cubic Birch-Murnaghan equation to determine bulk modulus (B(0)) and equilibrium volume (V(0)).
• Literature data analysis for comparative insights into hydrate mechanical properties.

Main Results:

• Unit cell parameters were measured for ethane (0-2 GPa), xenon (0-1.5 GPa), and tetrahydrofuran+xenon (0-3 GPa) hydrates.
• Bulk moduli were determined: ethane hydrate (11.2 GPa), xenon hydrate (9.3 GPa), and tetrahydrofuran+xenon hydrate (8.8 GPa up to 1.5 GPa).
• The bulk modulus of gas hydrates with classical polyhedral structures is approximately (9±2) GPa, mainly dependent on the water framework's elasticity.

Conclusions:

• The bulk modulus of gas hydrates is largely dictated by the hydrogen-bonded water framework.
• Guest molecule filling significantly influences the bulk modulus.
• A recommended bulk modulus of (9±2) GPa is proposed for hydrates with classical polyhedral structures up to 1.5 GPa.